A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate may be determined. This may be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometers are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Although scatterometry is a relatively quick form of analysis of a surface, measuring only the intensity of scattered radiation may not achieve a desired preciseness of measurement, as it may not take into account the different behavior of radiation that is polarized in various directions. For example, if the substrate object that is being measured is in the form of a grating that is aligned with one polarization direction, radiation polarized in that direction will scatter in a different manner from radiation polarized in the orthogonal direction. To take polarization directions into account, an ellipsometric system has been envisaged to measure certain parameters of orthogonally polarized beams.
FIG. 4 shows an example of an ellipsometric sensor (or an ellipsometer) that has been envisaged to take into account the above. Illumination radiation from source P is reflected from a structure 30 on a target portion of a substrate W and on its return journey from the substrate, it is linearly polarized along one of the two eigen-polarizations of three beamsplitters that are present in the sensor (the eigen-polarizations being measured with respect to the x- or y-direction as shown in FIG. 4). A first beamsplitter N-PBS reflects part of the illumination to two further beamsplitters: one beamsplitter 80 sends part of the illumination to an imaging branch; and another beamsplitter 82 sends part of the illumination to a focus branch. The first beamsplitter N-PBS is a non-polarizing beamsplitter that directs the rest of the beam to a camera CCD. Having passed through the non-polarizing beamsplitter N-PBS, the polarized beam passes through a phase modulator 90 whose ordinary and extraordinary axes have been positioned at 45° with respect to the x- and y-directions. Subsequently, the beam is divided into its respective x- and y-polarization orientations using a Wollaston prism 50 and impinges on a camera CCD. The relative intensities of the polarized beams are used to determine the relative polarization orientations of the different parts of the beam. From the relative polarization orientations, the effect of the structure 30 on the beam as a whole can be determined. From the effect that structure 30 has on the beam, the properties of the structure itself can be determined.
U.S. Pat. No. 5,880,838 to Marx et al., which is incorporated by reference herein in its entirety, also describes the measurement of a structure on a substrate using ellipsometry, wherein the measurement system is called polarization quadrature measurement (PQM). This document describes focusing a polarized beam of light (with transverse electric TE and transverse magnetic TM fields) onto the structure. The TM and TE fields are affected differently by the diffraction off the structure. The TE field may be used as a reference to analyze the phase and amplitude changes in the TM field. The relationship between phases and amplitudes of the TE and TM fields is dependent on the structural parameters (e.g., the depth of a hole or the height of a grating bar or the pitch of a grating) of the structure. By measuring this relationship, therefore, the structural parameters may be determined.
Rather than just measuring the intensity variation within an illumination beam, generally, ellipsometry can be used to measure the state of polarization of scattered light. Ellipsometry measures two parameters: a phase difference (Δ) between two differently polarized beams and an amplitude ratio (tan ψ) of two polarized beams. With these two parameters, any polarization state of a purely polarized beam may be described.
Specifically, if an incident beam has both s and p polarizations, the reflected beam will have reflectance coefficients Rp and Rs. Delta (Δ) is the phase difference between the reflectance coefficients Rp and Rs as given in equation (1) below.
The intensity of the received beam is proportional to the sum of the amplitudes, taking into account the angle of their relative polarization. For example, if the polarizations of both Rp and Rs are aligned in the same orientation, the intensity of the received beam is at a maximum. If the two amplitudes are in orthogonal orientations, they cancel each other out and the intensity is at a minimum. The angle between the two polarization directions (or orientations) is ψ and so the relationship between ψ and Rp and Rs can be defined by equation (2).Δ=arg(Rp−Rs)  (1)tan ψ=Rp/Rs  (2)
FIG. 8 shows the relationship between these two parameters. Specifically, FIG. 8 shows the intensity variation in one pixel as a function of phase difference between s and p that is imposed by phase modulator 90 of FIG. 4. I is the intensity of the beam and P is the overall polarization of Rp and Rs. Assuming the two amplitudes are the same (i.e., Rp=Rs and ψ=45°), the intensity of the overall beam is at a minimum at point x because the polarization directions cancel each other out. At point y, the intensity is at a maximum, indicating that the polarization directions are aligned.
The overall intensity shown in FIG. 8 is modulated, demonstrating that the amplitudes (being the same) cancel each other out to a greater or lesser extent and so the relative phase of the two beams can be monitored as changing accordingly (as dictated by the phase modulator 90).
A system such as that shown in FIG. 4, which incorporates a phase modulator 90 (or phase shifters), has the following specific features:
1. The phase shifts that are applied to the light may need to be known exactly because any inaccuracies in these phase shifts may result in the same inaccuracy in Δ. It is desirable to know the relationship between intensity and phase in order for the structure to be accurately determined.
2. Phase modulators are wavelength-dependent, which means that phase modulators may have to be recalibrated for each wavelength that is used.
3. With phase modulators, two or more phase shifts are applied to each beam of light at a specific wavelength. The intensities of the differently shifted beams may have to be re-measured for each shift, taking significant amounts of time.
4. Using a phase shifter for analysis of an object on a substrate means that an image of the object may need to be recorded for each change in phase, causing the data-gathering step to be longer than desired. This is not desirable when a quick analysis is desired so that subsequent substrates may be corrected if an error in alignment, for instance, is found.
Two potential solutions to the use of the phase modulator have been proposed. Both have, as their aim, obtaining four differently polarized reflected sub-beams from a single incident beam in order to measure, from a measured intensity of each sub-beam, a difference in amplitude and phase of the four known polarizations. The first potential solution obtains this result by having the reflected beam pass through at least two polarizing beamsplitters arranged at 90° with respect to each other such that a radiation beam is split into two orthogonally polarized sub-beams and each of those polarized sub-beams is subsequently split at a 90° angle into mutually orthogonally polarized sub-sub-beams. All four sub-beams are therefore at 0°, 90°, 180°, and 270° polarization angles (with respect to each other). Wollaston prisms and the like are also used to split the beam into sub-beams, and each beam polarized by a different angle. The second potential solution passes a beam through a single polarizing device that has four quadrants, each quadrant with a polarizer having a different polarizing angle such that the beam is effectively divided into four quadrants, each with polarization in a different direction (e.g., 0°, 45°, 135°, and 180°). In the potential solutions described above, separate sub-beams of different polarizations are compared using the same or different cameras and the effect of the object on the substrate is compared for the different polarization angles. Analysis of the image by the camera gives rise to the characteristics of the structure from which the radiation beam has been reflected.
However, the solutions described above incorporate several different devices, each of which may have to be calibrated and which may absorb a certain amount of the radiation beam each time the beam passes through it. Furthermore, several devices in series may exacerbate a small error in the beam diffracted from the structure.